Fuel cell electrode pair assemblies and related methods

ABSTRACT

Disclosed herein are fuel cell systems and, more specifically, fuel cell electrode pair and stack assemblies and various methods relating thereto. In one embodiment, the present invention is directed to a fuel cell electrode pair assembly adapted for use with a fuel cell system, wherein the electrode pair assembly comprises an anode structure derived from a first silicon substrate and an opposing cathode structure derived from a second silicon substrate, wherein at least (i) the anode structure comprises one or more discrete anodic porous active regions disposed across a top surface, or (ii) the cathode structure comprises one or more discrete cathodic porous active regions disposed across a top surface, and wherein the anode structure and the cathode structure each have at least one adjoining support member made of silicon, one or more plastics, or one or more glasses, and wherein the at least one adjoining support member of the anode structure and the at least one adjoining support member of the cathode structure have interfacing surfaces that are bonded together with an optional interposing binding material and with at least one selectively positioned bond to thereby form a hermetic seal, wherein the at least one selectively positioned bond is selected from the group consisting of a silicon-metal eutectic-silicon bond, a silicon-frit-silicon bond, a silicon-metal-silicon microwave bond, a silicon-polymer adhesive-silicon bond, a silicon-polymer adhesive-plastic bond, a silicon-polymer adhesive-glass bond, or a silicon-glass anodic bond.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/291,202 filed May 15, 2001, which provisionalapplication is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] The present invention relates generally to fuel cell systems and,more specifically, to fuel cell electrode pair and stack assemblies andvarious methods relating thereto.

BACKGROUND OF THE INVENTION

[0003] A fuel cell is an energy conversion device that consistsessentially of two opposing electrodes, an anode and a cathode,ionically connected together via an interposing electrolyte. Unlike abattery, fuel cell reactants are supplied externally rather thaninternally. Fuel cells operate by converting fuels, such as hydrogen ora hydrocarbon (e.g., methanol), to electrical power through anelectrochemical process rather than combustion. It does so by harnessingthe electrons released from controlled oxidation-reduction reactionsoccurring on the surface of a catalyst. A fuel cell can produceelectricity continuously so long as fuel is supplied from an outsidesource.

[0004] In electrochemical fuel cells employing methanol as the fuelsupplied to the anode (also commonly referred to as a “Direct MethanolFuel Cell (DMFC)” system), the electrochemical reactions are essentiallyas follows: first, a methanol molecule's carbon-hydrogen, andoxygen-hydrogen bonds are broken to generate electrons and protons;simultaneously, a water molecule's oxygen-hydrogen bond is also brokento generate an additional electron and proton. The carbon from themethanol and the oxygen from the water combine to form carbon dioxide.Oxygen from air (supplied to the cathode) is likewise simultaneouslyreduced at the cathode. The ions (protons) formed at the anode migratethrough the interposing electrolyte and combine with the oxygen at thecathode to form water. From a molecular perspective, the electrochemicalreactions occurring within a direct methanol fuel cell (DMFC) system areas follows: $\begin{matrix}{\quad {{Anode}\text{:}}} & {\quad {{{{CH}_{3}{OH}} + {H_{2}O}}->{{6H^{+}} + {6e^{-}} + {CO}_{2}}}} & {E_{0} = {0.04\quad V}} & {{vs}.} & {NHE} & (1) \\{\quad {{Cathode}\text{:}}} & {\quad {{{\frac{3}{2}O_{2}} + {6H^{+}} + {6e^{-}}}->{3H_{2}O}}} & {E_{0} = {1.23\quad V}} & {{vs}.} & {NHE} & (2) \\{\quad {{Net}\text{:}}} & {\quad {{{{CH}_{3}{OH}} + {\frac{3}{2}O_{2}}}->{{2H_{2}O} + {CO}_{2}}}} & {E_{0} = {1.24\quad V}} & {{vs}.} & {NHE} & (3)\end{matrix}$

[0005] The various electrochemical reactions associated with otherstate-of-the-art fuel cell systems (e.g., hydrogen or carbonaceous fuel)are likewise well known to those skilled in the art of fuel celltechnologies.

[0006] With respect to state-of-the-art fuel cell systems generally,several different configurations and structures have beencontemplated—most of which are still undergoing further research anddevelopment. In this regard, existing fuel cell systems are typicallyclassified based on one or more criteria, such as, for example: (1) thetype of fuel and/or oxidant used by the system, (2) the type ofelectrolyte used in the electrode stack assembly, (3) the steady-stateoperating temperature of the electrode stack assembly, (4) whether thefuel is processed outside (external reforming) or inside (internalreforming) the electrode stack assembly, and (5) whether the reactantsare fed to the cells by internal manifolds (direct feed) or externalmanifolds (indirect feed). In general, however, it is perhaps mostcustomary to classify existing fuel cell systems by the type ofelectrolyte (i.e., ion conducting media) employed within the electrodestack assembly. Accordingly, most state-of-the-art fuel cell systemshave been classified into one of the following known groups:

[0007] 1. Alkaline fuel cells (e.g., KOH electrolyte);

[0008] 2. Acid fuel cells (e.g., phosphoric acid electrolyte);

[0009] 3. Molten carbonate fuel cells (e.g., Li₂CO₃/K₂CO₃ electrolyte);

[0010] 4. Solid oxide fuel cells (e.g., yttria-stabilized zirconiaelectrolyte);

[0011] 5. Proton exchange membrane fuel cells (e.g., NAFIONelectrolyte).

[0012] Although these state-of-the-art fuel cell systems are known tohave many diverse structural and operational characteristics, suchsystems nevertheless share many common characteristics with respect tothe joining or bonding together of the individual electrode structuresthat form the electrode stack assembly. Put simply, most conventionalstate-of-the-art electrode stack assemblies consist essentially of aseries of conjoined anode and cathode structures, wherein the faces ofthe electrode structures (together with any separator and fluid flowplates) are adjacently positioned next to one another and attachedtogether by means of adhesives and/or bolted tie rods. Moreover, mostconventional fuel cell stack assemblies also include a plurality offluid tight resilient seals, such as elastomeric gaskets. The use ofsuch elastomeric gaskets (together with the disparate materials used forthe separator and fluid flow plates) necessitates the need to have aconstant compressive force applied along the longitudinal axis of thestack assembly to ensure resilient sealing. Hence, and in order tomaintain proper sealing between adjacent surfaces, conventional fuelcell stacks are generally compressed together by one or more metal tierods or tension members. In general, the end bolted tie rods or tensionmembers of such conventional state-of-the-art stack assemblies extendthrough holes formed in stack's end plates; in this configuration aconstant compressive force is maintained throughout the stack assembly.In addition, adhesives such as, for example, epoxides are often alsoapplied between the various opposing faces of the stack components toensure that the stack is hermetically sealed.

[0013] Exemplary fuel cell electrode stack assemblies in accordance withthe prior art are disclosed in U.S. Pat. No. 5,723,228 to Okamoto(discloses DMFC system having a series of bolted together membraneelectrode assemblies with interposing gaskets and separator plates), (2)U.S. Pat. No. 6,190,793 B1 to Barton et al. (discloses a fuel cell stackassembly having non-conductive tie rod tension members); and (3) U.S.Pat. No. 6,057,053 to Gibb (discloses compression assembly for a fuelcell stack). A significant problem associated with these conventionalfuel cell stack designs, however, is their limited ability to be scaleddown so as to be manufacturable on a micro-scale basis. In particular,these conventional fuel cell stack designs are not generally amenable tothe “stacking” of silicon and/or sol-gel derived electrode structures(which electrode structures are generally made by micro-fabricationtechniques and are associated with micro-scale fuel cell systems).Accordingly, there is still a need in the art for improved fuel cellelectrode stack assemblies, systems, and related methods. The presentinvention fulfills these needs and provides for further relatedadvantages.

SUMMARY OF THE INVENTION

[0014] In brief, the present invention relates generally to fuel cellsystems and, more specifically, to fuel cell electrode pair and stackassemblies and various methods relating thereto. In one embodiment, thepresent invention is directed to a fuel cell electrode pair assemblyadapted for use with a fuel cell system, wherein the electrode pairassembly comprises an anode structure derived from a first siliconsubstrate and an opposing cathode structure derived from a secondsilicon substrate, wherein at least (i) the anode structure comprisesone or more discrete anodic porous active regions disposed across a topsurface, or (ii) the cathode structure comprises one or more discretecathodic porous active regions disposed across a top surface, andwherein the anode structure and the cathode structure each have at leastone adjoining support member made of silicon, one or more plastics, orone or more glasses, and wherein the at least one adjoining supportmember of the anode structure and the at least one adjoining supportmember of the cathode structure have interfacing surfaces that arebonded together with an optional interposing binding material and withat least one selectively positioned bond to thereby form a hermeticseal, wherein the at least one selectively positioned bond is selectedfrom the group consisting of a silicon-metal eutectic-silicon bond, asilicon-frit-silicon bond, a silicon-metal-silicon microwave bond, asilicon-polymer adhesive-silicon bond, a silicon-polymeradhesive-plastic bond, a silicon-polymer adhesive-glass bond, or asilicon-glass anodic bond.

[0015] In some embodiments, the at least one adjoining support member ofthe anode structure or the at least one adjoining support member of thecathode structure is made of silicon that is integral to the anodestructure or the cathode structure. In other embodiments the at leastone adjoining support member of the anode structure and the at least oneadjoining support member of the cathode structure are both made ofsilicon, and wherein the optional binding material is a metal, andwherein the at least one selectively positioned bond is thesilicon-metal eutectic-silicon bond. Preferably, however, the metal isgold, tin, lead, copper, silver, aluminum, or a combination thereof, andmore preferably the metal is gold. In addition, the fuel cell electrodepair assembly may further comprise (i) a dielectric layer on at leastone of the interfacing surfaces, wherein the dielectric layer comprisessilicon dioxide, silicon nitride, or a combination thereof, and (ii) anadhesion layer on the dielectric layer, wherein the adhesion layercomprises titanium, chromium, tungsten, aluminum, or a combinationthereof. Preferably, however, the adhesion layer is a titanium-tungstenlayer.

[0016] In other embodiments, the at least one adjoining support memberof the anode structure and the at least one adjoining support member ofthe cathode structure are both made of silicon, and wherein the optionalbinding material is a frit, and wherein the at least one selectivelypositioned bond is the silicon-frit-silicon bond. Preferably, however,the frit comprises a silicate. In addition, the fuel cell electrode pairassembly may further comprise a dielectric layer on at least one of theinterfacing surfaces, wherein the dielectric layer comprises silicondioxide, silicon nitride, or a combination thereof.

[0017] In still other embodiments, the at least one adjoining supportmember of the anode structure and the at least one adjoining supportmember of the cathode structure are both made of silicon, and whereinthe optional binding material is a metal, and wherein the at least oneselectively positioned bond is the silicon-metal-silicon microwave bond.Preferably, however, the metal is gold, tin, lead, copper, silver,aluminum, or a combination thereof, and more preferably the metal isgold. In addition, the fuel cell electrode pair assembly may furthercomprise (i) a dielectric layer on at least one of the interfacingsurfaces, wherein the dielectric layer comprises silicon dioxide,silicon nitride, or a combination thereof, and (ii) an adhesion layer onthe dielectric layer, wherein the adhesion layer comprises titanium,chromium, tungsten, aluminum, or a combination thereof. Preferably,however, the adhesion layer is a titanium-tungsten layer.

[0018] In still further embodiments, the at least one adjoining supportmember of the anode structure and the at least one adjoining supportmember of the cathode structure are both made of silicon, and whereinthe anode structure and the cathode structure are made of differentmaterials, and wherein the optional binding material comprises a polymeradhesive, and wherein the at least one selectively positioned bond isthe silicon-polymer adhesive-silicon bond. In these embodiments, thepolymer adhesive may comprise one or more amorphous fluoropolymers,benzocyclobutane, polydimethylsiloxane, perfluoro2,2-dimethyl-1,3-dioxole, tetrafluoroethylene, or a combination thereof.

[0019] In still further embodiments, the at least one adjoining supportmember of the anode structure and the at least one adjoining supportmember of the cathode structure are each made of silicon or the one ormore plastics, and wherein the anode structure and the cathode structureare made of different materials, and wherein the optional bindingmaterial comprises a polymer adhesive, and wherein the at least oneselectively positioned bond is the silicon-polymer adhesive-plasticbond. In these embodiments, the one or more plastics are eachindependently selected from the group consisting of siloxanes, epoxies,polyimides, polyphenylene ethers, polyphenylene sulfides, polysufones,or fluoropolymers; and the polymer adhesive may comprise one or moreamorphous fluoropolymers, benzocyclobutane, polydimethylsiloxane,perfluoro 2,2-dimethyl-1,3-dioxole, tetrafluoroethylene, or acombination thereof.

[0020] In still further embodiments, the at least one adjoining supportmember of the anode structure and the at least one adjoining supportmember of the cathode structure are each made of silicon or the one ormore glasses, and wherein the anode structure and the cathode structureare made of different materials, and wherein the optional bindingmaterial comprises a polymer adhesive, and wherein the at least oneselectively positioned bond is the silicon-polymer adhesive-glass bond.In these embodiments, the one or more glasses may comprise aborosilicate glass. In addition, the polymer adhesive may comprise oneor more amorphous fluoropolymers, benzocyclobutane,polydimethylsiloxane, perfluoro 2,2-dimethyl-1,3-dioxole,tetrafluoroethylene, or a combination thereof.

[0021] In still further embodiments, the at least one adjoining supportmember of the anode structure and the at least one adjoining supportmember of the cathode structure are each made of silicon or the one ormore glasses, and wherein the anode structure and the cathode structureare made of different materials, and wherein the at least oneselectively positioned bond is the silicon-glass anodic bond. In theseembodiments, the one or more glasses may comprise a borosilicate glass.

[0022] These and other aspects of the present invention will become moreevident upon reference to the following detailed description andattached drawings. It is to be understood, however, that variouschanges, alterations, and substitutions may be made to the specific fuelcell electrode structures disclosed herein without departing from theessential spirit and scope of the present invention. In addition, it isto be further understood that the drawings are illustrative and symbolicrepresentations of exemplary embodiments of the present invention(hence, they are not necessarily to scale). Finally, it is expresslyprovided that all of the various references cited herein areincorporated by reference in their entireties.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1A illustrates a top plan view of an electrode structurehaving a plurality of acicular pores in accordance with an embodiment ofthe present invention.

[0024]FIG. 1B illustrates a cross-sectional view of the electrodestructure of FIG. 1A, wherein the view is taken along line B-B of FIG.1A.

[0025]FIG. 1C illustrates a top perspective view of the electrodestructure of FIGS. 1A and 1B.

[0026]FIG. 2A illustrates a cross-sectional view of an exemplaryelectrode assembly in accordance with an embodiment of the presentinvention, wherein a planar anode and a planar cathode have poroussilicon substrate regions, and wherein the planar anode and the planarcathode are attached to each other by a plurality of bridge members thatspan across a spaced apart region.

[0027]FIG. 2B illustrates a top view of the electrode assembly of FIG.2A.

[0028]FIG. 3 illustrates a cross-sectional view of an exemplary fuelcell electrode stack assembly in accordance with an embodiment of thepresent invention.

[0029]FIG. 4 illustrates an exploded perspective view of an exemplaryfuel cell electrode stack assembly in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0030] The present invention relates generally to fuel cell systems and,more specifically, to fuel cell electrode pair and stack assemblies andvarious methods relating thereto. As is appreciated by those skilled inthe art, a fuel cell system generally comprises a stack of electrodepair assemblies (referred to as an electrode stack assembly), whereineach individual electrode pair assembly consists essentially of twoopposing electrode structures, an anode and a cathode, ionicallyconnected together via an interposing electrolyte. The electrode stackassembly of such fuel cell systems also generally includes a series offlow channels for flowing reactant streams adjacent to and/or throughdiscrete regions of the electrode structures.

[0031] In the several embodiments set forth herein, the inventive fuelcell electrode pair and stack assemblies are based, in large part, onnovel electrode structures that are particularly useful for carrying acatalyst, wherein the catalyst is readily accessible to flowing gaseousand/or liquid reactant streams. In this regard, electrode structuresuseful for practicing the present invention principally include thosemade from silicon substrates such as, for example, silicon wafers. Inparticular, it has been discovered that electrode structures made fromsilicon wafers are particularly useful in miniature fuel cell systems(especially micro-scale direct methanol fuel cell systems), mainlybecause such electrode structures are able to provide a high surfacearea to bulk volume ratio, have good mechanical strength, and arecompatible with various thin/thick films which are often needed formaking selected electrical connections. Because of these physicalcharacteristic, among others, the electrode pair and stack assemblies ofthe present invention are capable of delivering reliable power.

[0032] Accordingly, and without limitation to any particularmethodology, the individual silicon electrode structures of the presentinvention may be made out of silicon wafers by utilizing standardmicroelectronic processes such as, for example, alkaline etching, plasmaetching, lithography, electroplating, as well as electrochemical poreformation. In this way, a silicon substrate useful for carrying acatalyst may be produced, wherein the silicon substrate may have anynumber of pores and pores sizes such as, for example, random or orderedpore arrays—including pore arrays having selected pore diameters,depths, and distances relative to one another. In short, the siliconelectrode structures of the present invention may have any number ofpossible porosities and/or void spaces associated therewith.

[0033] An exemplary embodiment of a porous silicon electrode structureuseful as a component of the present invention is shown in FIGS. 1A-C,which drawings show an isolated electrode structure 200 (which structureis adapted for use with a fuel cell system). The electrode structure 200of this embodiment comprises a silicon substrate 210 (thickness rangingfrom about 300 to about 500 microns) having one or more discrete porousregions 220 disposed across a top surface 230 of the substrate 210. Inaddition, each of the one or more discrete porous regions 220 is definedby a plurality of acicular or columnar pores 240 that extend through thesubstrate 210 (diameter ranging from about 0.5 to about 10 microns). Theplurality of acicular or columnar pores define inner pore surfaces 250,and the inner pore surfaces 250 may have an optional conformalelectrically conductive layer 270 thereon. In some embodiments and asshown, the pores are substantially perpendicular to the top and bottomsurfaces 230, 235 of the substrate 210. In some other embodiments, thepores each have a diameter of about 10 microns and are spaced apart fromone another about 10 microns (from pore center axis to adjacent porecenter axis) so as to yield substrate having an approximate 39%porosity.

[0034] Porous silicon electrode structures useful in the context of thepresent invention may be formed by silicon micro-machining and/or wetchemical techniques (employed by the semiconductor industry) such as,for example, anodic polarization of silicon in hydrofluoric acid. As isappreciated by those skilled in the art, the anodic polarization ofsilicon in hydrofluoric acid (HF) is a chemical dissolution techniqueand is generally referred to as HF anodic etching; this technique hasbeen used in the semiconductor industry for wafer thinning, polishing,and the manufacture of thick porous silicon films. (See, e.g., Eijkel,et al., “A New Technology for Micromachining of Silicon: DopantSelective HF Anodic Etching for the Realization of Low-DopedMonocrystalline Silicon Structures,” IEEE Electron Device Ltrs.,11(12):588-589 (1990)). In the context of the present invention, it isto be understood that the porous silicon may be nanoporous silicon(i.e., average pore size <2 nm), mesoporous silicon (i.e., average poresize of 2 nm to 50 nm), or macroporous silicon (i.e., average poresize >50 nm); the pores may also be a series of parallelly alignedacicular or columnar pores that extend into or through the siliconsubstrate. Although the pores may be angled, they are preferablysubstantially perpendicular to the surfaces of the substrate.

[0035] More specifically, porous silicon electrodes useful in thecontext of the present invention may be formed by a photoelectrochemicalHF anodic etching technique, wherein selected oxidation-dissolution ofsilicon occurs under a controlled current density. (See, e.g.,Levy-Clement et al., “Porous n-silicon Produced by PhotoelectrochemicalEtching,” Applied Surface Science, 65/66: 408-414 (1993); M. J. Eddowes,“Photoelectrochemical Etching of Three-Dimensional Structures inSilicon,” J. of Electrochem. Soc., 137(11):3514-3516 (1990).) Anadvantage of this relatively more sophisticated technique over others isthat it is largely independent of the different principalcrystallographic planes associated with single-crystal silicon wafers(whereas most anisotropic wet chemical etching methods have verysignificant differences in rates of etching along the differentprincipal crystallographic planes).

[0036] In the context of the present invention, an electrode pairassembly adapted for use with a fuel cell system generally comprises: ananode structure derived from a first silicon substrate, an interposingelectrolyte, and an opposing cathode structure derived from a secondsilicon substrate. The anode structure is generally processed so as tohave one or more discrete anodic porous active regions disposed across atop surface of the anode, wherein each of the one or more discreteanodic porous active regions is defined by a plurality of anodicacicular pores that extend into or through the anode. Similarly, thecathode structure is generally processed so as to have one or morediscrete cathodic active porous regions disposed across a top surface ofthe cathode, wherein each of the one or more discrete porous cathodicporous active regions is defined by a plurality of cathodic acicularpores that extend into or through the cathode. The opposing anode andcathode structures are generally spaced apart and substantially parallelto each other so as to define a spaced apart region, and the electrolyteis generally interposed between the anodic porous active region and thecathodic porous active region.

[0037] In addition, the electrode pair assembly may further comprise afluid delivery channel disposed across a first face of the anodestructure or the cathode structure; and a fluid removal channel disposedacross a second face of the anode structure or the cathode structure;wherein at least one of the one or more discrete anodic porous activeregions or at least one of the one or more discrete cathodic activeporous regions is (i) interposed between the fluid delivery channel andthe fluid removal channel, and (ii) adapted to flow a liquidtherebetween (e.g., the acicular or columnar pores serve as hydrodynamictransport channels or fluid flow through-holes). In such aconfiguration, a solid polymer electrolyte (e.g., NAFION, E.I DuPont deNemours, United States) or a flowing fluid reactant/electrolyte mayionically connect the anode to the cathode. Suitable fluidreactant/electrolytes include, for example, an organic liquid fuelcombined with an acid solution (i.e., a liquid aprotic organicelectrolyte). Exemplary organic fuels include alcohols such as methanol,ethanol, and propanol, or a combination thereof; and the acid solutionmay be phosphoric acid, sulfuric acid, or an organic sulfonic acid suchas trifluormethanesulfonic acid and its derivatives, or a combinationthereof. In some embodiments, the fluid reactant/electrolyte comprises amixture of methanol and water with an acid, where acid concentration isabout 0.5 M to 12 M, and preferably about 4 M. The bonding methods usedto assemble individual electrode structures into an electrode pair orstack assembly ensure that interfacing electrode structures are isolatedin a manner that provides electrolyte and fuel distribution to theactive regions, as well as separation from the oxidant supply. Moreover,the bonding methods disclosed herein allow for the formation of hermeticand water tight seals.

[0038] Thus, and in accordance with the embodiment represented by FIGS.2A and 2B, an electrode assembly 300 useful for practicing the presentinvention comprises a planar anode structure 302 made from a siliconsubstrate, an electrolyte layer 304, a planar cathode structure 306 madefrom a silicon substrate, and optionally a blocking layer 308 that issubstantially impermeable to at least methanol and is substantiallypermeable to protons. As shown, the planar anode structure 302 and theplanar cathode structure 306 are spaced apart and substantially parallelto each other so as to define a spaced apart region 310, wherein theelectrolyte layer 304 and optional blocking layer 308 are interposedbetween the planar anode structure 302 and the planar cathode structure306 and within at least a portion of the spaced apart region 310, andwherein the planar anode structure 302 and the planar cathode structure306 are attached to each other by at least one bridge member 312 thatspans across the spaced apart region 310. As depicted, fuel (optionallycombined with a liquid electrolyte) flows through the anode and, in someembodiments, through the spaced apart region 310; an oxidant such as,for example, air, oxygen, or a hydrogen peroxide solution simultaneouslyflows through the cathode. As also shown, the anode structure 302 andthe cathode structure 306 each have opposing faces that are conjoinedtogether with at least one selectively positioned bond selected from thegroup consisting of a silicon-metal eutectic-silicon bond, asilicon-frit-silicon bond, a silicon-metal-silicon microwave bond, asilicon-polymer adhesive-silicon bond.

[0039] With respect to electrode pair assemblies having at least oneselectively positioned silicon-metal eutectic-silicon bond, it is to beunderstood that opposing silicon electrode structures may be fusionbonded together by heating and compressing together individual siliconelectrode structures having one or more metals deposited thereon at atemperature that is at or above the eutectic point of the silicon-metalbinary system (i.e., the silicon substrate and the immediately adjacentmetal layer). In this way, the metal component is able to partiallydiffuse into the underlying crystalline silicon matrix of the opposingsilicon fuel cell electrode structure; and upon cooling, the partiallycommingled and solidified lattice structures of the silicon substratesand adjacent metal layer are understood to be fusion bonded togetherwith a silicon-metal eutectic-silicon bond.

[0040] In general, the method for making an electrode pair or stackassembly in accordance with this embodiment of the present inventionfirst involves depositing a dielectric layer on one of the two faces ofthe opposing silicon electrode structures. The dielectric layer servestwo purposes: (1) it prevents electron transport or cross-over betweenadjacently positioned silicon electrode structures; and (2) it preventsdiffusion/migration of the metal component into the silicon electrodestructure. In other words, the dielectric layer functions as a barrierlayer. As is appreciated by those skilled in the art, suitabledielectrics for these purposes include, for example, silicon oxide,silicon nitride, a polyimide, or a glass. Preferably, however, thedielectric layer is either a deposited or grown (e.g., thermally grown)silicon oxide layer that has a thickness ranging from about 1,000 to10,000 Angstroms, or a deposited (e.g., LPCVD) silicon nitride layerthat has a thickness ranging from about 600 to 1,000 Angstroms.

[0041] In order to enhance adhesion of the deposited metal component,the method may optionally comprise depositing an adhesion layer on thedielectric layer of each of the silicon electrode structures (of theelectrode pair or stack assembly) so as to form a plurality of siliconelectrode structures each having an exposed silicon face and an adhesionface. The adhesion layer may comprise titanium, chromium, tungsten,aluminum, or a combination thereof. Preferably, however, the adhesionlayer comprises a deposited (e.g., sputtered) titanium-tungsten alloylayer that has a thickness ranging from about 150 to 1,000 Angstroms,wherein the titanium-tungsten alloy consist essentially of about 5weight percent titanium and about 95 weight percent tungsten.

[0042] Following deposition of the dielectric layers and optionaladhesion layers, the method further comprises depositing a metal on atleast a portion of the dielectric layer (or adhesion layer) of each ofthe plurality of silicon electrode structures. As with the dielectriclayer, the metal layer also serves two purposes: (1) it allows forfusion bonding between opposing silicon electrode structures; and (2) itoptionally allows for electron transport through the stack assembly. Asis appreciated by those skilled in the art, suitable metals for thesepurposes include, for example, gold, tin, lead, copper, silver,aluminum, or a combination thereof. Preferably, however, the metal isdeposited (e.g., sputtered) as a gold layer having a thickness rangingfrom about 0.5 to 5 microns.

[0043] Next, the silicon electrode structures are spatially aligned suchthat the faces of each of the plurality of silicon electrode structuresare substantially parallel to one another, and such that the exposedsilicon faces are immediately adjacent to the metal (e.g., gold)deposited on the dielectric faces (or adhesion faces). The plurality ofsilicon electrode structures are then moved together such that they comeinto contact with one another; heated for first selected period of timeand to a temperature that is at or above the eutectic point of themetal/silicon binary system. This assembly is then compressed togetherwith a selected force and for a second selected period of time. Finally,the assembly is allowed to cool such that silicon-metal eutectic-siliconbonds are formed between each of the plurality of silicon electrodestructures thereby yielding the electrode pair or stack assembly inaccordance with this embodiment of the present invention. As isappreciated by those skilled in the art, there are a number of bondingapparatuses or jigs capable of spatially aligning, moving, heating, andcompressing the plurality of silicon electrode structures together.

[0044] In addition, and depending on the type of metal deposited andbonding apparatus employed, the following parameters are considered tobe typical: the temperature associated with the step of heating theplurality of silicon electrode structures that are in contact with oneanother generally ranges from about 373° C. to 450° C.; the firstselected period of time associated with the step of heating theplurality of silicon electrode structures that are in contact with oneanother generally ranges from about 1 to 30 minutes; the forceassociated with the step of compressing the plurality of siliconelectrode structures together generally ranges from about 7 to 700 kPa;and the second selected period of time associated with the step ofcompressing the plurality of silicon electrode structures togethergenerally ranges from about 1 to 15 minutes.

[0045] With respect to electrode pair assemblies having at least oneselectively positioned silicon-frit-silicon bond, it is to be understoodthat opposing silicon electrodes structures may be fusion bondedtogether by heating and compressing together individual siliconelectrode structures (optionally having one or more metals depositedthereon) at a temperature that is at or above the melting point of thefrit. As is appreciated by those skilled in the art, a “frit” is asilicate, glassy or glassy-crystalline sintering or fusion product madeof a mixture of glass-forming and/or glass affecting materials such as,for example, quartz, feldspar, clays, borax, alkali metal and alkalineearth metal carbonates. In this way, the frit component is able topartially diffuse into the underlying crystalline silicon matrix of thesilicon fuel cell electrode structure; and upon cooling, the partiallycommingled and solidified lattice structures of the silicon substrateand adjacent frit layer are understood to be fusion bonded together witha silicon-frit-silicon bond.

[0046] In general, the method for making an electrode pair or stackassembly in accordance with this embodiment involves essentially thesame steps as the above-described silicon-metal eutectic-silicon bondingmethod, except that a frit paste is used in lieu of the metal layer. Inthis regard, the frit paste is generally deposited by means of screenprinting as a layer having a thickness ranging from about 5 to 100microns. The solvent associated with the frit paste may then beevaporated by placing each pasted electrode structure onto an 80-120° C.hot plate for about 1-15 minutes, thereby yielding a frit layer. Thefrit layer may then be cured by exposure to O₂ at a temperature rangingfrom about 300-550° C. for about 10-30 minutes. Next, the plurality ofsilicon electrode structure are spatially aligned such that the faces ofeach of the plurality of silicon electrode structures are substantiallyparallel to one another, and such that the exposed silicon faces areimmediately adjacent to the deposited frit layer. The plurality ofsilicon electrode structures are then moved together such that they comeinto contact with one another; heated for first selected period of timeand to a temperature that is at or above the melting point of the fritlayer. This assembly is then compressed together with a selected forceand for a second selected period of time. Finally, the assembly isallowed to cool such that silicon-frit-silicon bonds are formed betweeneach of the plurality of silicon electrode structures thereby yieldingthe electrode pair or stack assembly in accordance with this embodimentof the present invention.

[0047] With respect to electrode pair assemblies having at least oneselectively positioned silicon-metal-silicon microwave bond, it is to beunderstood that opposing silicon electrode structures may be fusionbonded together by applying microwave energy to adjacently positionedsilicon electrode structures having one or more metals depositedthereon. As is appreciated by those skilled in the art, silicon, quartz,certain ceramics and plastics are transparent to microwave energy; andas such, a deposited interposing metal (such as, for example, indium,aluminum, titanium, tin, nickel, gold, or a combination thereof)deposited in between interfacing silicon electrode structures(optionally having an adjoining support member made of silicon, one ormore plastics, or one or more glasses) may be selectively heated andmelted. In this way, silicon-metal-silicon microwave bonds may be formedto yield an electrode pair or stack assembly in accordance with thisembodiment of the present invention.

[0048] With respect to electrode pair assemblies having at least oneselectively positioned silicon-polymer adhesive-silicon bond or at leastone selectively positioned silicon-polymer adhesive-plastic bond, it isto be understood that opposing silicon electrode structures (wherein oneof the electrode structures may have an adjoining support member made ofplastic or a glass), may be bonded together by compressing together(optionally with heat) individual electrode structures (optionallyhaving one or more metals deposited thereon) that have had a polymeradhesive (i.e., interposing binding material) applied as a coating to atleast one of the interfacing electrode structure surfaces. In thisregard, the polymer adhesive may be polydimethylsiloxane (PDMS) orbenzocyclobutane (BCB); alternatively, the polymer adhesive may be oneor more amorphous fluoropolymers such as, for example, TEFLON (DuPontFluoroproducts, U.S.A.). As is appreciated by those skilled in the art,TEFLON represents a family of amorphous copolymers based onperfluoro(2,2-dimethyl-1,3-dioxole) (PPD) with other fluorine-containingmonomers. The polymer adhesives of the present invention are generallymixed with a suitable carrier solvent such as, for example, xylene andan optional photoinitiator such as, for example, dimethoxy phenylacetophenone (DMAP) prior to application.

[0049] In general, the method for making an electrode pair or stackassembly in accordance with this embodiment involves spin coating thepolymer adhesive with solvent onto at least one of the interfacingelectrode structure surfaces. In the case where the polymer adhesive ispolydimethylsiloxane (PDMS), the coating may be exposed to ulraviolent(UV) light having an approximate wavelength of about 420 nm by use ofstandard photolithography techniques. Next, the coating is brieflyexposed to O₂ plasma. The plasma treated surfaces bond together uponcontact with one another at room temperature to thereby yield anelectrode pair or stack assembly in accordance with this embodiment ofthe present invention; namely, electrode pair or stack assemblies havinga silicon-polymer adhesive-silicon bond, a silicon-polymeradhesive-plastic bond, or a silicon-polymer adhesive-glass bond.

[0050] Finally, and with respect to electrode pairs having at least oneselectively positioned silicon-glass anodic bond, it is to be understoodthat opposing silicon electrode structures (wherein one of the electrodestructures has an adjoining support member made of a glass) may befusion bonded together by anodically bonding together adjacentlypositioned silicon electrode structures wherein one of the interfacingsurfaces is silicon and the other is a glass (e.g., a borosilicateglass). As is appreciated by those skilled in the art, anodic bondingrefers to bonding of silicon to silicon or silicon to glass by use of anapplied electric field.

[0051] In view of the foregoing and with reference to FIGS. 3 and 4, afuel cell electrode stack assembly 400 in accordance with the presentinvention may be made by bonding together a plurality of electrode pairassemblies 402, wherein each electrode pair assembly 402 comprises ananode structure 404 derived from a first substrate and an opposingcathode structure 406 derived from a second substrate. The first andsecond substrates are preferably derived from one or more siliconwafers; however, it is to be appreciated that other materials such asporous carbon, raney nickel, and a sol-gel are all possible. As shown inFIG. 3, each anode structure 404 and each cathode structure 406 may haveat least one adjoining support member 408 that is made of silicon, oneor more of the plastics disclosed herein, or one or more of the glassesdisclosed herein. In addition, the at least one adjoining support member408 of the anode structure 404 and the at least one adjoining supportmember 408 of the cathode structure 406 may each have interfacingsurfaces 410, 412 that are bonded together with a first optionalinterposing binding material 414 (as disclosed herein) to thereby form ahermetic or water tight seal.

[0052] As noted above, the interfacing surfaces 410, 412 between theadjoining support members 408 together with the first optionalinterposing binding material 414 is understood to include at least oneselectively positioned bond selected from the group consisting of asilicon-metal eutectic-silicon bond, a silicon-frit-silicon bond, asilicon-metal-silicon microwave bond, a silicon-polymer adhesive-siliconbond, a silicon-polymer adhesive-plastic bond, a silicon-polymeradhesive-glass bond, or a silicon-glass anodic bond as disclosed herein.Similarly, the interfacing surfaces 416, 418 between the adjoiningsupport members 408 and the anode structure 404, as well as theinterfacing surfaces 420, 422 between the adjoining support members 408and the cathode structure 406, may include, depending on the material ofthe support members 408 and second optional interposing binding material424, at least one selectively positioned bond selected from the groupconsisting of a silicon-metal eutectic-silicon bond, asilicon-frit-silicon bond, a silicon-metal-silicon microwave bond, asilicon-polymer adhesive-silicon bond, a silicon-polymeradhesive-plastic bond, a silicon-polymer adhesive-glass bond, or asilicon-glass anodic bond as disclosed herein.

[0053] Finally, the fuel cell electrode stack assembly 400 may alsoinclude a pair of end plates 426 bonded together to the plurality ofelectrode pair assemblies 402 by means of a third optional bindingmaterial 428 (as disclosed herein)., Each end plate 426 may be made ofsilicon, one or more of the plastics disclosed herein, or one or more ofthe glasses disclosed herein. Accordingly, the interfacing surfaces 430,432 between the pair of end plates 426 and the plurality of electrodepair assemblies 402 may include, depending on the material of the endplates 426 and the third optional interposing binding material 428, atleast one selectively positioned bond selected from the group consistingof a silicon-metal eutectic-silicon bond, a silicon-frit-silicon bond, asilicon-metal-silicon microwave bond, a silicon-polymer adhesive-siliconbond, a silicon-polymer adhesive-plastic bond, a silicon-polymeradhesive-glass bond, or a silicon-glass anodic bond as disclosed herein.

[0054] For purposes of illustration and not limitation, the followingexamples more specifically disclose various aspects of the presentinvention.

EXAMPLES

[0055] Examples 1-8 disclose, among other things, general processingsteps associated with making electrode stack pairs and assemblies inaccordance with certain embodiments of the present invention.

Example 1 Stack Assembly of Silicon Electrode Structures Having Au—SiEutectic Fusion Bonds

[0056] This example discloses the processing steps associated withmaking a silicon-based electrode stack assembly adapted for use with afuel cell system in accordance with an embodiment of the presentinvention. In this example, the processing steps assume thatpre-fabricated anode and cathode silicon electrode structures have beenprovided, wherein one of the two opposing faces of each electrodestructure already has a dielectric layer formed thereon. Accordingly,the processing steps consist essentially of assembling a plurality ofelectrode structures together to form an electrode pair or stack.

[0057] Electrode Stack Assembly Fabrication

[0058] Note that the stack configuration can be cathode-anode orcathode-anode-anode-cathode. In either case the terminus is a completecell.

[0059] 1.1 Nitride Removal—Use a reactive ion etcher with an SF₆ plasma(may use a CHF₃ and O₂ plasma) to remove nitride from the front and backfaces of the electrodes.

[0060] 1.2 Oxide Removal—Use a reactive ion etcher with a CHF₃ and O₂plasma (may use an SF₆ plasma) to remove oxide from the surfaces onwhich an Au-Si eutectic will be formed. The oxide will not be removedfrom surfaces which will have Au deposited on them.

[0061] 1.3 Deposit Gold—First deposit a 500 Å Ti layer on the oxidelayers in an electron beam evaporator. Then deposit 2 μm Au on top ofthe Ti in an electron beam evaporator.

[0062] 1.4 Eutectic Formation—Align the wafers and bring them intocontact. Apply pressure in the range of 7 to 700 kPa and heat to 373 to450° C. for 1 to 10 minutes.

[0063] 1.5 Dicing—Dice the bonded wafers along the dicing lane to yieldsingle stacks of electrodes.

Example 2 Stack Assembly of Sol-Gel Derived Electrode Structures HavingAu—Si Eutectic Fusion Bonds

[0064] This example discloses the processing steps associated withmaking a sol-gel derived electrode stack assembly adapted for use with afuel cell system in accordance with an embodiment of the presentinvention. In this example, the processing steps assume thatpre-fabricated anode and cathode sol-gel electrode structures have beenprovided, wherein one of the two opposing faces of each electrodestructure already has a dielectric layer formed thereon. Accordingly,the processing steps consist essentially of assembling a plurality ofelectrode structures together to form an electrode pair or stack.Without limitation, the principal processing steps are set forth below.

Electrode Stack Assembly Fabrication

[0065] Note that the stack configuration can be cathode-anode orcathode-anode-anode-cathode. In either case the terminus is a completecell.

[0066] 1.1 Bonding—Align and contact together the wafers and end caps.Heat to 373-450° C. at 7-700 kPa for 1-10 minutes.

[0067] 1.2 Electrolyte Injection—Introduce NAFION into the electrodestack through the anode and cathode flow channels.

[0068] 1.3 Dicing—Dice the wafer into separate electrode stacks.

[0069] 1.4 Solder—Deposit gold solder on the edge of the stack forelectrical contact.

Example 3 Thermal Compression Bonding of Electrode Structures with aFrit Paste

[0070] This example discloses the processing steps associated withmaking an electrode pair or stack assembly adapted for use with a fuelcell system in accordance with an embodiment of the present invention.In this example, the processing steps assume that pre-fabricated anodeand cathode electrode structures have been provided (silicon-basedoptionally comprising a sol-gel), wherein one of the two opposing facesof each electrode structure already has a dielectric layer formedthereon. Accordingly, the processing steps consist essentially ofassembling a plurality of electrode structures together to form anelectrode pair or stack. Without limitation, the principal processingsteps are set forth below.

[0071] Electrode Stack Assembly Fabrication

[0072] Note that the stack configuration can be cathode-anode orcathode-anode anode-cathode. In either case the terminus is a completecell. Moreover, the thermal compression bonding aspect generally doesnot require wafer polishing as the presence of oxide or nitride layersare generally not detrimental to the bond strength.

[0073] 1.1 Prepare a frit paste (e.g., a mixture of powdered glass, abinder such as ethyl cellulose, and a solvent such as turpineol).

[0074] 1.2 Screen print the paste onto the bonding faces of eachelectrode structure so as to yield a 5 to 100 μm thick layer.

[0075] 1.3 Allow the paste to dry by placing each pasted electrodestructure onto an 80-120° C. hot plate for about 1-15 minutes (this stepevaporates most of the solvent; hence, the temperature employed dependsupon the solvent used in paste).

[0076] 1.4 Burn off the remaining solvent in a 300-400° C. furnace, 20minutes.

[0077] 1.5 Burn off the binder in a 375-425° C. furnace, 20 minutes.

[0078] 1.6 Melt glass frit in a 350-550° C. furnace, 30 minutes(temperature depends on the type of glass used in the initial fritpaste).

[0079] 1.7 Allow to cool, then bring the glass surfaces into contactwith one another.

[0080] 1.8 Apply 7-700 kPa and fire in 400-850° C. furnace, 30 minutes,thereby yielding the electrode stack assembly.

Example 4 Electrode Pair Structures with a Silicon-Metal-SiliconMicrowave Bond

[0081] This example discloses the processing steps associated withmaking an electrode pair or stack assembly adapted for use with a fuelcell system in accordance with an embodiment of the present invention.In this example, the processing steps assume that pre-fabricated anodeand cathode electrode structures have been provided (silicon-basedoptionally comprising a sol-gel), wherein one of the two opposing facesof each electrode structure already has a dielectric layer formedthereon. Accordingly, the processing steps consist essentially ofassembling a plurality of electrode structures together to form anelectrode pair or stack. Without limitation, the principal processingsteps are set forth below.

[0082] Note that the stack configuration can be cathode-anode orcathode-anode-anode cathode. In either case the terminus is a completecell.

[0083] 1.1 Deposit Gold—Deposit a 500 Å Ti layer then a 1200 Å Au layeron each opposing face of the electrode in an electron beam evaporator.

[0084] 1.2 Bonding—Align and contact together the wafers and end caps.Place in chamber of a microwave bonder and expose to 2.45 GHz microwavesfor 3-30 seconds.

[0085] 1.3 Dicing—Dice the wafer into separate electrode stacks.

[0086] 1.4 Solder—Deposit gold solder on the edge of the stack forelectrical contact.

Example 5 Electrode Pair Structures With A Silicon-PolymerAdhesive-Silicon Bond

[0087] This example discloses the processing steps associated withmaking an electrode pair or stack assembly adapted for use with a fuelcell system in accordance with an embodiment of the present invention.In this example, the processing steps assume that pre-fabricated anodeand cathode electrode structures have been provided (silicon-basedoptionally comprising a sol-gel), wherein one of the two opposing facesof each electrode structure already has a dielectric layer formedthereon. Accordingly, the processing steps consist essentially ofassembling a plurality of electrode structures together to form anelectrode pair or stack. Without limitation, the principal processingsteps are set forth below.

[0088] Note that the stack configuration can be cathode-anode orcathode-anode-anode cathode. In either case the terminus is a completecell.

[0089] 1.1 Prepare Polymer Spin-on Solution—Prepare 1 weight % dimethoxyphenyl acetophenone (DMAP) in polydimethylsiloxane (PMDS) spin-onsolution.

[0090] 1.2 Deposit PMDS Film—Using a spin coater at 3000 rpm for 15 to45 seconds, apply 5 milliliters PMDS spin-on solution to the opposingfaces.

[0091] 1.3 Cure PMDS Film—Expose the PMDS film to ultraviolet light at420 nanometers wavelength for 5 to 120 seconds.

[0092] 1.4 Plasma Treatment—Expose the cured PMDS films to O₂ plasma at0.2 Torr and 25 W for 20 seconds in a reactive ion etcher.

[0093] 1.5 Bonding—Align and contact together the wafers and end caps.Allow to set for 24 hours.

[0094] 1.6 Dicing—Dice the wafer into separate electrode stacks.

[0095] 1.7 Solder—Deposit gold solder on the edge of the stack forelectrical contact.

Example 6 Electrode Pair Structures With A Silicon-PolymerAdhesive-Plastic Bond

[0096] This example discloses the processing steps associated withmaking an electrode pair or stack assembly adapted for use with a fuelcell system in accordance with an embodiment of the present invention.In this example, the processing steps assume that pre-fabricated anodeand cathode electrode structures have been provided (silicon-basedoptionally comprising a sol-gel), wherein one of the two opposing facesof each electrode structure already has a dielectric layer formedthereon. Accordingly, the processing steps consist essentially ofassembling a plurality of electrode structures together to form anelectrode pair or stack. Without limitation, the principal processingsteps are set forth below.

[0097] Note that the stack configuration can be cathode-anode orcathode-anode-anode cathode. In either case the terminus is a completecell.

[0098] 1.1 Prepare Polymer Spin-on Solution—Prepare 1 weight % dimethoxyphenyl acetophenone (DMAP) in polydimethylsiloxane (PMDS) spin-onsolution.

[0099] 1.2 Deposit PMDS Film—Using a spin coater at 3000 rpm for 15 to45 seconds, apply 5 milliliters PMDS spin-on solution to the opposingfaces.

[0100] 1.3 Cure PMDS Film—Expose the PMDS film to ultraviolet light at420 nanometers wavelength for 5 to 120 seconds.

[0101] 1.4 Plasma Treatment—Expose the cured PMDS films to O₂ plasma at0.2 Torr and 25 W for 20 seconds in a reactive ion etcher.

[0102] 1.5 Bonding—Align and contact together the wafers and end caps.Allow to set for 24 hours.

[0103] 1.6 Dicing—Dice the wafer into separate electrode stacks.

[0104] 1.7 Solder—Deposit gold solder on the edge of the stack forelectrical contact.

Example 7 Electrode Pair Structures with a Silicon-PolymerAdhesive-Glass Bond

[0105] This example discloses the processing steps associated withmaking an electrode pair or stack assembly adapted for use with a fuelcell system in accordance with an embodiment of the present invention.In this example, the processing steps assume that pre-fabricated anodeand cathode electrode structures have been provided (silicon-basedoptionally comprising a sol-gel), wherein one of the two opposing facesof each electrode structure already has a dielectric layer formedthereon. Accordingly, the processing steps consist essentially ofassembling a plurality of electrode structures together to form anelectrode pair or stack. Without limitation, the principal processingsteps are set forth below.

[0106] Note that the stack configuration can be cathode-anode orcathode-anode-anode cathode. In either case the terminus is a completecell.

[0107] 1.1 Prepare Polymer Spin-on Solution—Prepare 1 weight % dimethoxyphenyl acetophenone (DMAP) in polydimethylsiloxane (PMDS) spin-onsolution.

[0108] 1.2 Deposit PMDS Film—Using a spin coater at 3000 rpm for 15 to45 seconds, apply 5 milliliters PMDS spin-on solution to the opposingfaces.

[0109] 1.3 Cure PMDS Film—Expose the PMDS film to ultraviolet light at420 nanometers wavelength for 5 to 120 seconds.

[0110] 1.4 Plasma Treatment—Expose the cured PMDS films to O₂ plasma at0.2 Torr and 25 W for 20 seconds in a reactive ion etcher.

[0111] 1.5 Bonding—Align and contact together the wafers and end caps.Allow to set for 24 hours.

[0112] 1.6 Dicing—Dice the wafer into separate electrode stacks.

[0113] 1.7 Solder—Deposit gold solder on the edge of the stack forelectrical contact.

Example 8 Electrode Pair Structures with a Silicon-Glass Anodic Bond

[0114] This example discloses the processing steps associated withmaking an electrode pair or stack assembly adapted for use with a fuelcell system in accordance with an embodiment of the present invention.In this example, the processing steps assume that pre-fabricated anodeand cathode electrode structures have been provided (silicon-basedoptionally comprising a sol-gel), wherein one of the two opposing facesof each electrode structure already has a dielectric layer formedthereon. Accordingly, the processing steps consist essentially ofassembling a plurality of electrode structures together to form anelectrode pair or stack. Without limitation, the principal processingsteps are set forth below.

[0115] Note that the stack configuration can be cathode-anode orcathode-anode-anode cathode. In either case the terminus is a completecell.

[0116] 1.1 Nitride Removal—Use a reactive ion etcher with an SF₆ plasma(may use a CHF₃ and O₂ plasma) to remove nitride from the front and backfaces of the electrodes.

[0117] 1.2 Oxide Removal—Use a reactive ion etcher with a CHF₃ and O₂plasma (may use an SF₆ plasma) to remove oxide from the surfaces onwhich an anodic bond.

[0118] 1.3 Bonding—Align and contact together one silicon wafer and oneborosilicate glass wafer. Place the pair on a planar positive electrodesuch that the silicon wafer is in contact with the electrode, then placethis assembly on a 350-500° C. hot plate. Place a point negativeelectrode on the borosilicate glass wafer and apply 800-1250 volts for15 to 30 minutes.

[0119] 1.4 Dicing—Dice the wafer into separate electrode stacks.

[0120] 1.5 Solder—Deposit gold solder on the edge of the stack forelectrical contact.

[0121] While the present invention has been described in the context ofthe embodiments illustrated and described herein, the invention may beembodied in other specific ways or in other specific forms withoutdeparting from its spirit or essential characteristics. Therefore, thedescribed embodiments are to be considered in all respects asillustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription, and all changes that come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

We claim:
 1. A fuel cell electrode pair assembly adapted for use with afuel cell system, wherein the electrode pair assembly comprises an anodestructure derived from a first silicon substrate and an opposing cathodestructure derived from a second silicon substrate, wherein at least (i)the anode structure comprises one or more discrete anodic porous activeregions disposed across a top surface, or (ii) the cathode structurecomprises one or more discrete cathodic porous active regions disposedacross a top surface, and wherein the anode structure and the cathodestructure each have at least one adjoining support member made ofsilicon, one or more plastics, or one or more glasses, and wherein theat least one adjoining support member of the anode structure and the atleast one adjoining support member of the cathode structure haveinterfacing surfaces that are bonded together with an optionalinterposing binding material and with at least one selectivelypositioned bond to thereby form a hermetic seal, wherein the at leastone selectively positioned bond is selected from the group consisting ofa silicon-metal eutectic-silicon bond, a silicon-frit-silicon bond, asilicon-metal-silicon microwave bond, a silicon-polymer adhesive-siliconbond, a silicon-polymer adhesive-plastic bond, a silicon-polymeradhesive-glass bond, or a silicon-glass anodic bond.
 2. The fuel cellelectrode pair assembly of claim 1 wherein the at least one adjoiningsupport member of the anode structure or the at least one adjoiningsupport member of the cathode structure is made of silicon that isintegral to the anode structure or the cathode structure.
 3. The fuelcell electrode pair assembly of claim 1 or 2 wherein the at least oneadjoining support member of the anode structure and the at least oneadjoining support member of the cathode structure are both made ofsilicon, and wherein the optional binding material is a metal, andwherein the at least one selectively positioned bond is thesilicon-metal eutectic-silicon bond.
 4. The fuel cell electrode pairassembly of claim 3 wherein the metal is gold, tin, lead, copper,silver, aluminum, or a combination thereof.
 5. The fuel cell electrodepair assembly of claim 3 wherein the metal is gold.
 6. The fuel cellelectrode pair assembly of claim 3, further comprising a dielectriclayer on at least one of the interfacing surfaces.
 7. The fuel cellelectrode pair assembly of claim 6 wherein the dielectric layercomprises silicon dioxide, silicon nitride, or a combination thereof. 8.The fuel cell electrode pair assembly of claim 6, further comprising anadhesion layer on the dielectric layer.
 9. The fuel cell electrode pairassembly of claim 8 wherein the adhesion layer comprises titanium,chromium, tungsten, aluminum, or a combination thereof.
 10. The fuelcell electrode pair assembly of claim 8 wherein the adhesion layer is atitanium-tungsten layer.
 11. The fuel cell electrode pair assembly ofclaim 1 or 2 wherein the at least one adjoining support member of theanode structure and the at least one adjoining support member of thecathode structure are both made of silicon, and wherein the optionalbinding material is a frit, and wherein the at least one selectivelypositioned bond is the silicon-frit-silicon bond.
 12. The fuel cellelectrode pair assembly of claim 11 wherein the frit comprises asilicate.
 13. The fuel cell electrode pair assembly of claim 11, furthercomprising a dielectric layer on at least one of the interfacingsurfaces.
 14. The fuel cell electrode pair assembly of claim 13 whereinthe dielectric layer comprises silicon dioxide, silicon nitride, or acombination thereof.
 15. The fuel cell electrode pair assembly of claim1 or 2 wherein the at least one adjoining support member of the anodestructure and the at least one adjoining support member of the cathodestructure are both made of silicon, and wherein the optional bindingmaterial is a metal, and wherein the at least one selectively positionedbond is the silicon-metal-silicon microwave bond.
 16. The fuel cellelectrode pair assembly of claim 15 wherein the metal comprises indium,aluminum, titanium, tin, nickel, gold, or a combination thereof.
 17. Thefuel cell electrode pair assembly of claim 15, further comprising adielectric layer on at least one of the interfacing surfaces.
 18. Thefuel cell electrode pair assembly of claim 17 wherein the dielectriclayer comprises silicon dioxide, silicon nitride, or a combinationthereof.
 19. The fuel cell electrode pair assembly of claim 17, furthercomprising an adhesion layer on the dielectric layer.
 20. The fuel cellelectrode pair assembly of claim 19 wherein the adhesion layer comprisestitanium, chromium, tungsten, aluminum, or a combination thereof. 21.The fuel cell electrode pair assembly of claim 1 wherein the at leastone adjoining support member of the anode structure and the at least oneadjoining support member of the cathode structure are both made ofsilicon, and wherein the anode structure and the cathode structure aremade of different materials, and wherein the optional binding materialcomprises a polymer adhesive, and wherein the at least one selectivelypositioned bond is the silicon-polymer adhesive-silicon bond.
 22. Thefuel cell electrode pair assembly of claim 21 wherein the polymeradhesive comprises one or more amorphous fluoropolymers.
 23. The fuelcell electrode pair assembly of claim 21 wherein the polymer adhesivecomprises benzocyclobutane, polydimethylsiloxane, perfluoro2,2-dimethyl-1,3-dioxole, tetrafluoroethylene, or a combination thereof.24. The fuel cell electrode pair assembly of claim 1 wherein the atleast one adjoining support member of the anode structure and the atleast one adjoining support member of the cathode structure are eachmade of silicon or the one or more plastics, and wherein the anodestructure and the cathode structure are made of different materials, andwherein the optional binding material comprises a polymer adhesive, andwherein the at least one selectively positioned bond is thesilicon-polymer adhesive-plastic bond.
 25. The fuel cell electrode pairassembly of claim 24 wherein the one or more plastics are eachindependently selected from the group consisting of siloxanes, epoxies,polyimides, polyphenylene ethers, polyphenylene sulfides, polysufones,or fluoropolymers.
 26. The fuel cell electrode pair assembly of claim 24wherein the polymer adhesive comprises one or more amorphousfluoropolymers.
 27. The fuel cell electrode pair assembly of claim 24wherein the polymer adhesive comprises benzocyclobutane,polydimethylsiloxane, perfluoro 2,2-dimethyl-1,3-dioxole,tetrafluoroethylene, or a combination thereof.
 28. The fuel cellelectrode pair assembly of claim 1 wherein the at least one adjoiningsupport member of the anode structure and the at least one adjoiningsupport member of the cathode structure are each made of silicon or theone or more glasses, and wherein the anode structure and the cathodestructure are made of different materials, and wherein the optionalbinding material comprises a polymer adhesive, and wherein the at leastone selectively positioned bond is the silicon-polymer adhesive-glassbond.
 29. The fuel cell electrode pair assembly of claim 28 wherein theone or more glasses comprises borosilicate glass.
 30. The fuel cellelectrode pair assembly of claim 27 wherein the polymer adhesivecomprises one or more amorphous fluoropolymers.
 31. The fuel cellelectrode pair assembly of claim 27 wherein the polymer adhesivecomprises benzocyclobutane, polydimethylsiloxane, perfluoro2,2-dimethyl-1,3-dioxole, tetrafluoroethylene, or a combination thereof.32. The fuel cell electrode pair assembly of claim 1 wherein the atleast one adjoining support member of the anode structure and the atleast one adjoining support member of the cathode structure are eachmade of silicon or the one or more glasses, and wherein the anodestructure and the cathode structure are made of different materials, andwherein the at least one selectively positioned bond is thesilicon-glass anodic bond.
 33. The fuel cell electrode pair assembly ofclaim 32 wherein the one or more glasses comprises borosilicate glass.